The invention relates to drive assembly for a four wheel drive vehicle in the case of which the wheels of the front axle are driven permanently and the wheels of the rear axle are driven by a viscous coupling arranged in the longitudinal driveline. A freewheeling unit is connected in series with the viscous coupling and, during forward driving, permits the rear wheels to rotate faster than the front wheels.
From U.S. Pat. No. 3,760,922 it is known to provide drive assemblies with viscous couplings having at least two sets of plates with one set each being associated with a coupling hub and a coupling housing. The remaining space between the alternately arranged set of plates is partially filled with a viscous fluid, e.g. silicone oil.
In the case of drive assemblies for four wheel drive vehicles provided with viscous couplings, it is temporarily necessary to disconnect the rear axle from the engine drive, for example when operating the vehicle brake in order to maintain the stability of the vehicle.
From DE 33 17 247 A1 it is known, for example, to provide the rear axle with a torque derived from the front axle differential of a vehicle permanently driven via the front wheel axle. A viscous coupling is incorporated into the longitudinal driveline of such a vehicle, so that if a speed differential between the two wheel axles occurs, an increasing driving torque is transmitted to the rear wheels. A speed differential between the two axles occurs of necessity for example if the permanently driven front wheels are subject to a higher degree of slip. As a result, an additional torque builds up for the rear axle because the speed differential between the two axles causes a relative movement of the sets of plates relative to each other, with the viscous fluid in the coupling housing being sheared. Under normal driving conditions this effect is desirable.
However, the disadvantage of this design of a viscous coupling in the driveline is that independently of how the speed differential occurs, the viscous coupling changes into the torque transmitting condition as soon as a speed differential occurs. If the vehicle is braked, the shearing effect of the viscous fluid also causes a braking moment to be transmitted to the rear axle via the viscous coupling, so that an increased slip occurs at the rear axle, with the vehicle losing its lateral stability.
Even with vehicles with an anti-locking device, this disadvantage is particularly noticeable.
To avoid such a critical driving condition, it is known to design the viscous coupling in such a way that, if necessary, it may be disconnected via a switching assembly. If the rear axle is subject to a higher speed than the front axle, the switching assembly acts like a freewheeling unit and prevents the occurrence of such critical driving conditions when the front axle is over-braked or the vehicle is subject to a load change.
For this reason, the patent specification already mentioned proposes a permanently effective freewheeling unit with a freewheeling torque being transmitted from the front axle to the rear axle during forward driving. During reversing under four wheel drive conditions, the freewheeling unit is blocked manually or automatically, for example by engaging the reverse gear. The disadvantage of this design refers to the mechanical blocking of the freewheeling unit between the front and rear axle during reversing. Furthermore, it is necessary to provide a switching device for operating the freewheeling lock, which renders the production of such a drive assembly more expensive.
From DE 37 08 193 A1 it is known to provide a permanently ineffective freewheeling device with a freewheeling lock which, when operating the vehicle brake, automatically unlocks the freewheeling lock and switches the freewheeling unit into an operative condition. However, the disadvantage of this design are that the freewheeling device can only be switched via the vehicle brake and that the rear axle cannot be disconnected when changing to a traction mode of operation.